Conventional laser diode drive circuits, such as the one disclosed in U.S. Pat. No. 6,392,215 issued May 21, 2002 in the name of Baumgartner et al and illustrated in FIG. 1, utilize a feedback loop 10 to control the bias current Ilaser, which drives the laser diode 12. The feedback loop 10 includes a monitor diode 16, which produces a monitor current Imon proportional to the power output of the laser diode 12. The monitor current Imon is mirrored via current mirror 18, which is comprised of transistors 26 and 28, and compared to a predetermined reference current Iref, which is generated by current source 22, and the result of this comparison is fed via lead 32 to an operational amplifier 20, which outputs a bias control signal 30. The bias control signal 30 directs a bias current source 14 to raise, lower or maintain the bias current Ilaser depending on whether more, less or the same amount of power is required from the laser diode 12. A compensating capacitor 24 is provided for filtering power supply noise. Unfortunately, the design of current comparators can be relatively complicated. Moreover, the prior art drive circuits do not include safety features to protect against unsafe levels of laser power, particularly redundant safety features dependent upon various electrical signals used in the drive circuit to ensure laser diode shutdown when undesired levels are detected.
An object of the present invention is to overcome the shortcomings of the prior art by providing a laser diode driving circuit utilizing voltage comparators instead of current comparators.
Another object of the present invention is to provide a laser diode driving circuit with safety features for ensuring that the laser diode operates within standard safety limits.